In 1976, a student at the Texas A&M University-Kingsville was tasked with feeding the snakes in what is now the university’s National Natural Toxins Research Center. Thinking it didn’t make much difference what kind of rodent the serpents ate, the student offered one of the Center’s western diamondback rattlesnakes a plump, fuzzy woodrat. The snake attacked by sinking its fangs into the rat’s fur—a maneuver that’s usually followed by near-instant hemorrhaging, clotting in the liver, and cardiac arrest.
But instead of keeling over, the rodent stared back at the God of Death and whispered:
Now, the woodrat is no honey badger, the snake-defying creature crowned by the internet as the namesake of casual badassery. Woodrats weigh less than a pound, have neither large claws nor crazy fangs, and certainly don’t look capable of tangoing with a full-grown rattlesnake. And yet, when the scientists repeated the trial, they found that the woodrats not only held their own against the rattlers, the rodents sometimes scratched and bit the snakes... to death. Woodrats, as it turns out, are immune to rattlesnake venom.
Makes sense, right? If you’re a cute little ball of fur that lives in rattlesnake country, then it’d sure be swell to be able to take a bite or two and still make it to hot yoga on time. For woodrats, venom immunity is like having a can of Fix-A-Flat in the car: You hope you never need it, but it’s handy in a pinch.
Woodrat: 1. Snake: 0.
The scorpion’s sting. The rattlesnake’s bite. The jellyfish’s slimy embrace. We humans spend a lot of time standing in awe and fear of the world’s most venomous creatures. Which makes sense: Any animal that can kill with little more than a prick of the skin is worthy of our respect. But there is a whole other class of creatures that does not cower before the venomous villains of the wild. These are the venom-immune. And they don’t give a cuss.
In fact, numerous critters have shown a honey-badger-like moxie when it comes to weathering the effects of chemical weapons. In the mammalian realm, hedgehogs, skunks, ground squirrels, and pigs have shown resistance to venom. Some scientists even believe the lowly opossum, which wields a venom-neutralizing peptide in its blood, may hold the key to developing a universal antivenom. Egyptian mongooses may be even more venom-indifferent than opossums, but alas, their protections don’t seem to be transferable. (Instead of antivenin blood, mongooses possess mutations on their very cells that block snake neurotoxins like a wad of gum in a keyhole.)
In this venom-fighting menagerie, woodrats are an exception of sorts. “Venom resistance is expensive and only works on certain predators, while other adaptations might be cheaper,” explains Christie Wilcox, author of the new book Venomous: How Earth’s Deadliest Creatures Mastered Biochemistry. In fact, venom resistance is far more common in those who eat venomous animals than those who venomous animals regularly feed upon.
For instance, grasshopper mice can shrug off the paralyzing effects of bark scorpions, upon which they feast. Same goes for the fan-fingered geckoes of the Middle East and the yellow scorpions they hunt. And Texas horned lizards are 1300 times more resistant to harvester ants than mice, a general indicator of toxicity.
Why would venom resistance evolve in predators more often than prey? Well, think of it this way: if you’re a predator, the number of dishes you can partake in at the buffet gets a lot larger if you can eat the spicy dishes everyone else is afraid of. You just have to figure out a way to neutralize the venomous animal’s hot sauce. Not only might this mean the difference between satiation and starvation, but it could be the difference between reproducing once versus four times over the course of the animal’s life.
“Those are big fitness consequences,” says Danielle Drabeck, an evolutionary biologist at the University of Minnesota and lead author of a Toxicon paper in 2015 that investigated the origins of honey badger immunity.
Plus, when you get right down to it, venomous creatures are pretty wimpy. “Snakes are limbless, small-boned, little bags of meat,” says Drabeck. “Even venomous snakes only have one pointy-end.” The same goes for cone snails, wasps, jellyfish, ants—take away their magical weapons and they’re almost pitiful. (OK, scorpions could still pinch, but that makes them about as formidable as a hermit crab.)
Besides mammals and lizards, there are plenty of snakes that are immune to snake venom. In some cases, it may be that immunity prevents the serpents from inadvertently committing suicide when they miss a mouse and hit themselves instead. (You know what it’s like to bite your cheek while you’re eating? Now imagine you’re venomous.) But in other cases, immunity points towards ophiophagy, or snake-eating.
“Venomous snakes exist in ecosystems as both predators and prey,” says Drabeck, “and in truth we understand relatively little about how their role as prey has shaped their evolution.” In fact, it’s entirely possible that snake venom first evolved as a defense mechanism: “Are the predatory uses of venoms more of a bonus side effect than their most important evolutionary purpose?” asks Wilcox, who is also a biologist studying jellyfish venoms at the University of Hawaii’s Pacific Cnidaria Research Laboratory.
“These are the kinds of questions that keep venom scientists up at night.”
By the way, you don’t need to have a fancy molecular mechanism to defeat venomous creatures. Some creatures just evolved really, really thick skin. (The honey badger has both: molecular defenses against cobra venom's neurotoxins, and loose, thick skin to help it avoid getting struck in the first place.)
Just look at leatherback sea turtle, says Wilcox. These oceanic behemoths make a living slurping up super-venomous jellyfish, stinging tentacles and all, like it ain’t no thing. As far we know, leatherbacks are not immune to the jellyfish’s sting. It’s just that they never get stung. From their shells and scaly skin to an esophagus that looks like the business end of a Sarlacc, the turtles have evolved countermeasures that prevent jellyfish from delivering their microscopic venom harpoons.
Of course, all of the turtle’s defenses might be rendered moot if it ate a poisonous animal as opposed to a venomous one. But that’s a whole other story.